Polyphenylene resins from vinylethynylbenzene, diethynylbenzene and phenylacetylene

ABSTRACT

A novel polyphenylene resin is formed by the polycyclotrimerization of vinylethynylbenzene, diethynylbenzene and phenylacetylene. Polymerization is carried out under conditions which result in the retention of unreacted vinyl groups from the vinylethynylbenzene component, which are then susceptible to crosslinking. The polymer is useful in the formation of high-performance carbon--carbon composites, producing an unusually high char yield with advantageous handling and processing characteristics.

This invention relates to polyphenylene resins for use in forminghigh-density carbon materials.

BACKGROUND OF THE INVENTION

High-density carbon-carbon composites are extremely strong materialscapable of withstanding high temperatures. Such composites find use inthe manufacture of structural parts for high-performance use,high-performance coatings, semi-conductor encapsulators andhigh-performance insulators. Examples of structural parts made fromthese materials are as heat shields for reentry vehicles and solidpropellant rocket motor nozzles.

These composites are formed from resins, phenolic resins being ingeneral use at present, by pyrolysis. Certain qualities of the resinsare significant in terms of their processing and performance as well asthe characteristics of the final product. These resin qualities includesolubility, flow characteristics, and char yield upon pyrolysis of theresin, as well as mechanical properties of the final product. A highchar yield, or low volatiles content, is particularly important, sinceit relates to the minimization of weight loss shrinkage, pores andcracking upon graphitization.

The phenolic resins in current use generally have char yields of lessthan 50%, due to the release of such decomposition products as water,carbon monoxide, phenol and methane upon pyrolysis and carbonization.With such a high quantity of volatiles produced, the resulting compositeis porous, low in density and susceptible to stress due to matrixshrinkage. To compensate for these deficiencies, the composite afterfirst having been formed is impregnated with coal tar pitch andrepyrolysed (i.e., "densified"). Five to seven densification cycles aregenerally required to achieve a product with thermostructural propertiesadequate for high performance use.

SUMMARY OF THE INVENTION

Novel resins based on aromatic acetylene hydrocarbon structures areprovided herein. These resins are generally prepared from a combinationof three monomers--(a) a vinylethynylbenzene, (b) a diethynylbenzene,and (c) phenylacetylene, with structural formulas as follows: ##STR1##The phenylacetylene functions solely as a chain terminator, whereas thevinylethynylbenzene functions as both a chain terminator and a providerof crosslinking sites. The monomers are polymerized in such a manner asto leave at least a substantial portion of the vinyl groups of thevinylethynylbenzene unreacted, preferably at least about 50%, mostpreferably at least about 75%. The relative amounts of the monomers arepreferably selected to achieve average molecular weights ranging fromabout 2,000 to about 100,000, preferably from about 5,000 to about50,000.

The resin offers a number of advantageous properties which distinguishit from both phenolic resins commonly used in carbon-composite materialsand other polyphenylene resins. Included among these properties are thefact that the resin is formed by relatively low-temperaturepolymerization, and once formed is soluble in common solvents. Inaddition, the resin crosslinks at a relatively low temperature, andproduces an unusually high char yield upon pyrolysis. The result uponcarbonization and graphitization is a product which needs littledensification, and has unusually high structural integrity. The highlyaromatic nature of the resin further imparts thermal and chemicalstability during processing. The absence of heteroatoms furthercontributes to the ease of processing, by reducing the presence ofimpurities which interfere with densification.

The high char yield causes very low shrinkage of the final product. Thispermits use of the resin in forming parts and components which cannot beformed using the conventional materials such as the phenolic resins. Airframe components are one example. The high char yield also imparts veryhigh strength to the product.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

In accordance with the present invention, the resin is formed by thepolycyclotrimerization of the three monomers. The structure of theresulting polymer will depend on the relative amounts of the monomers aswell as the polymerization conditions. A typical structure, however,will consist of phenyl rings linked together by single bonds betweenvertices of adjoining rings. The linked ring structure will includerings linked to one other ring, as well as rings linked to two otherrings and rings linked to three other rings. For rings bearing multiplelinkages, the linkages may be at adjacent vertices on the ring, or atvertices separated by one or more unlinked vertices. The linked ringstructure will also include vinyl groups (--CH═CH₂) bonded to ringvertices through single bonds. These groups are referred to herein as"terminal" vinyl groups. A typical structure is as follows: ##STR2##

The vinylethynylbenzene and diethynylbenzene are conveniently preparedas a mixture. As stated above, the ratio of these two components willvary depending on the characteristics sought for the resin which resultsupon polymerization. In most cases, a mixture in which the diethynylcomponent comprises from about 30% to about 85% by weight will providethe best results, with about 50% to about 75% by weight preferred. Thepositions of the ring substituents may be either ortho-, meta- or para-with respect to each other. The meta- and para- isomers are preferred.Particularly preferred is a mixture of meta- and para- isomers, sincesuch a mixture promotes increased solubility of the resulting resin inorganic solvents, and increased flexibility of the resulting polymers inall stages tip to carbonization. Technical grade divinylbenzene isparticularly useful for the preparation of the mixture of (a) and (b).This material is a mixture of divinyl and vinylethylbenzenes with minoramounts of diethylbenzenes and naphthalene. The divinylbenzenes arepresent in greater amounts than the vinylethylbenzenes, and themeta-isomers predominate.

The monomer mixture of the preceding paragraph (components (a) and (b)above) may be conveniently prepared from a mixture of divinylbenzene andethylvinylbenzene by bromination followed by dehydrobromination,according to conventional procedures. Addition of bromine to thecarbon--carbon double bonds occurs rapidly at room temperature in carbontetrachloride solution. Bromination at the ethyl group may beaccomplished by the use of a free-radical initiator or by reflexing incarbon tetrachloride, the free-radical reaction occurring more rapidly.Dehydrobromination is then achieved by the use of a non-nucleophilicbase, such as potassium t-butoxide or a phase transfer catalyst withpowdered potassium hydroxide. The product may then be extracted withhexane, then purified by distillation or chromatography.

The phenylacetylene functions solely as a chain terminator to controlthe molecular weight of the resulting polymer, and properties associatedtherewith. Further control of the molecular weight may be achieved byusing a combination of phenylacetylene with vinylacetylene, rather thanphenylacetylene alone. As stated above, the relative proportion of chainterminator with respect to the total monomer composition will varydepending on the desired characteristics of the resulting polymer. Therelative amounts may be expressed as a ratio of monofunctional todifunctional monomers, the monofunctional monomers including thevinylethynyl monomer (monomer (a) above) as well as the phenylacetylene(and vinylacetylene, when included). The ratio may vary, provided thatit is high enough to prevent the formation of a gel, yet low enough toavoid products with low melting points and poor thermal stability. Inmost cases, a ratio of monofunctional species to difunctional speciesranging from about 1 to about 2 will provide the best results.

Conditions for the polymerization reaction will be selected to retain asmany vinyl groups as possible.

Polymerization may be effected by the action of a Ziegler-type catalyst.Preferred Ziegler catalysts are complexes of titanium and aluminumcompounds, particularly titanium halides and alkyl aluminum compounds.The preferred catalyst is a complex of titanium tetrachloride and atrialkyl aluminum, with the alkyl group being either ethyl or isobutyl.

The reaction may be conducted over a broad temperature range, althoughhigher temperatures will generally leave fewer available vinyl groups inthe resulting polymer, and are less desirable for this reason. Apreferred temperature range is from about -10° C. to about 40° C. Whileother reaction conditions may vary, the reaction is generally conductedunder an inert atmosphere, at atmospheric pressure. Upon completion, thereaction may be quenched with acid.

At the completion of the reaction, the polymer may be isolated andpurified in accordance with conventional techniques. For example, theproduct solution may be purified by extraction with water and theproduct then precipitated in methanol to yield a cream-colored or yellowpowder. Further purification and fractionation may be achieved by theportionwise addition of methanol to a toluene solution of the polymer.

In its preferred form, the polymer will have a molecular weight rangingfrom about 2,000 to about 100,000, preferably from about 5,000 to about50,000, with a softening point ranging from about 200° C. to about 300°C.

Once formed, the polymer is preferably dissolved in a solvent tofacilitate processing. Conventional organic solvents may be used. Aparticularly preferred solvent, however, is the monomer mixture referredto above, which can be conveniently formed from technical gradedivinylbenzene. This solvent is preferred for purposes of furtherpolymerization, since it introduces no heteroatoms. Preferred solutionsare those in which the polymer comprises from about 25% to about 75% byweight, with about 40% to about 60% by weight particularly preferred.

The polymer may be processed into high-performance structural materialsin accordance with conventional techniques used in formingcarbon--carbon composites. In accordance with such techniques, a carbonfiller, such as finely divided graphite, for example, is generally usedto increase the resin density, toughness and char yield. Typicalprocedures for cure and pyrolysis would be curing at a temperature ofabout 100°-200° C., carbonization at a temperature of about 800°-1,000°C., then forming the carbonized resin into the desired structure,followed by graphitization at 1800°-2500° C.

The following examples are offered for purposes of illustration, and arenot intended to limit the invention in any manner.

EXAMPLE 1

A. Preparation of Monomer Mixture

To a 1-liter, round bottomed flask fitted with an addition funnel,thermometer, stirrer, an ice-water cooling bath and a N₂ purge connectedto a caustic trap, was charged 500 mL of carbon tetrachloride and 100 gof technical-grade divinylbenzene (analysis shown in Table 1). Thesolution was cooled to 10° C., and bromine (256 g, 1.6 mol) was addeddropwise, maintaining the reaction temperature at less than 20° C. bycontrolling the rate of addition. Benzoyl peroxide (5.0 g, 20.6 mmol)was added, and the mixture slowly heated to reflux (79° C.). (Caution:HBr evolved). The process was monitored by capillary column gaschromatography, and when conversion toα-bromoethyl-1,2-dibromoethylbenzene reached 99%, heating wasdiscontinued. After cooling to ambient, the reaction mixture was washedwith 100 mL of 25% caustic solution and then twice with 250 mL aliquotsof water. The solution was dried (MgSO₄) and stripped to yield 317 g(102%) of yellow solid.

To a 3-liter, round bottomed flask fitted with high speed agitation, areflux condensor, thermometer and static N₂ head was charged 600 mL ofpetroleum ether, 135 g of brominated divinylbenzene, 6.6 g (0.012 mol)of tetraoctyl ammonium bromide and 316 g (5.6 mol) of powdered potassiumhydroxide. The rapidly stirred mixture was heated to reflux (75°-80° C.)until the reaction was complete (˜3 hours) as shown by capillary columngas chromatography. The mixture was filtered, the filter cake washedwith 100 mL of petroleum ether, and the combined filtrates stripped toyield 31.2 g (88% yield) of pale yellow liquid.

                  TABLE 1                                                         ______________________________________                                        Capillary Column Gas Chromatograph                                            Area Percent Analysis of Technical-Grade                                      Divinylbenzene and Monomer Mixture                                            Starting              Monomer                                                 Material   Area %     Mixture     Area %                                      ______________________________________                                        m-divinyl  41.2       m-diethynyl 46                                          p-divinyl  15.4       p-diethynyl 15                                          m-vinylethyl                                                                             30.9       m-vinylethynyl                                                                            29                                          p-vinylethyl                                                                             8.5        p-vinylethynyl                                                                             9                                          m-diethyl  0.9        m, p-divinyl                                                                              <1                                          p-diethyl  0.9                                                                naphthalene                                                                              1.6        naphthalene <1                                          ______________________________________                                    

B. Preparation of Vinylpolyphenylene

To a dry 5-liter, round bottomed flask fitted with an addition funnel,stirrer, thermometer, a static N₂ head, and a rubber septum for additionvia a syringe, was charged 1.5 L of toluene, 114 mL (0.114 mol) oftitanium tetrachloride 1.0 molar in toluene, and 342 mL (0.342 mol) oftri-isobutylaluminum 1.0 molar in toluene. The mixture was cooled to 5°C., and a solution of 150 g of the monomer mixture of part A above(0.714 mol diethynylbenzene and 0.460 mol vinylethynylbenzene), 100 g(0.969 mol) of phenylacetylene in 50 mL of toluene, was added over aone-hour period, maintaining the temperature below 25° C. The solutionwas post-stirred for one hour, then quenched by the addition of 8 mL ofconcentrated hydrochloric acid in 75 mL of methanol. A thick brownslurry resulted from the quench. The slurry was washed with two 1-literaliquots of water, then 1 L of 6N hydrochloric acid, and finally 1 L ofwater. The thin toluene-product slurry was then added to 6 L of methanol(containing 3 mL of concentrated hydrochloric acid) which precipitatedthe vinylpolyphenylene as a yellow solid. The mixture was filtered, andthe filter cake washed with 2L of methanol. The product was dried invacuum at 50° C. to constant weight to give 237.4 g (95.0% yield).

Product analysis yielded the following results:

Vinyl content: Bromine titration showed 1.81 mmol olefin per gram (98%vinyl retention of polymer)

Ash content: 0.84 weight %

Molecular weight: ˜24000, range 68,000 to 1,000. Based on polystyrenestandard.

Softening point, °C.: 202°-240°

Solubility: 0.45 g/g toluene at 30° C.; 0.62 g/g toluene at 50° C.

Thermal stability tests yielded the following results:

                  TABLE 2                                                         ______________________________________                                        Thermal Stability                                                             Post-cure                      Weight                                         Time at Solubility Temperature Retention (%)                                  325° C.,                                                                       in Toluene,                                                                              of 10% wt.  At                                             hours   g/g @ 30° C.                                                                      loss, °C.                                                                          600° C.                                                                      1000° C.                          ______________________________________                                        0       0.45       455         78.0  --                                       6       insoluble  495         74.0  --                                       30      insoluble  527         84.2  66.6                                     ______________________________________                                    

EXAMPLE 2

Part B of Example 1 was repeated, using diisobutylaluminum chloride inplace of the tri-isobutylaluminum. Molecular weight determination andbromine titration revealed that all vinyl groups were incorporated intothe polymerization, leaving no vinyl groups available for crosslinking.

Part B of Example 1 was again repeated, using the catalyst systemindicated in that example, varying the temperature. Vinyl groupretention was determined by bromine titration, with the results shown inTable 3.

                  TABLE 3                                                         ______________________________________                                        Vinyl Content vs. Reaction Temperature                                        Temperature   Vinyl Content                                                   (°C.)  (% of original)                                                 ______________________________________                                         2            96                                                               4            80                                                              24            82                                                              36            50                                                              46            48                                                              ______________________________________                                    

EXAMPLE 3

Thermal and free-radical crosslinking of the vinyl groups were comparedin terms of the thermal stability of the resulting product. Thermalcuring under argon at 325° C. for 30 hours was shown to increase thetemperature of 10% weight loss (as determined by thermal gravimetricanalysis, or "TGA") from an initial 425° C. to 527° C. and to convertthe readily soluble polymer (45 weight % in toluene at 30° C.) to atotally insoluble cross-linked material. These data, tabulated below,show that: (1) vinyl crosslinking has occurred, and (2) suchcrosslinking increases thermal stability.

                  TABLE 4                                                         ______________________________________                                        Effect of Thermal Cure on Vinylpolyphenylene                                  Stability and Solubility                                                                   Toluene Solu-                                                                             Temperature of                                       Time at      bility at 30° C.,                                                                  10% Wt. Loss,                                        325° C., Hr.                                                                        Wt. %       °C.                                           ______________________________________                                        0            45          425                                                  6            insoluble   495                                                  30           insoluble   527                                                  ______________________________________                                    

Use of free-radical initiators (benzoyl peroxide and1,1'-azobis(cyclohexane)carbonitrile) were also successful incross-linking the vinyl groups. The resulting product again showedmarked insolubility and increased thermal stability to the 500°-550° C.range.

EXAMPLE 4

In a series of runs, part B of Example 1 was repeated with varyingratios of the phenylacetylene to the monomer mixture. The results arelisted in Table 5, where the ratio is expressed as a mole ratio ofmonofunctional to difunctional species, the former representing bothphenylacetylene and vinylethynylbenzene. In Run 1, the ratio is thatinherent in the monomer mixture itself (no phenylacetylene was used atall). In Run 5, the phenylacetylene was replaced by a mixture ofphenylacetylene and vinylacetylene at a mole ratio of 1.6. Run 4indicates that a mono- to difunctional ratio of 10 (usingphenylacetylene as chain terminator) produces a polymer with fewterminal vinyl groups available for crosslinking, with low melting pointand low thermal stability.

                  TABLE 5                                                         ______________________________________                                        Variation of Ratio of Chain Terminator to Monomer                                  Mole                                                                          Ratio                      Solubility                                         Mono-to  Average   Melting (g/g   Vinyl                                       Difunc-  Molecular (Softening)                                                                           toluene                                                                              Content                                Run  tional   Weight    Point, °C.                                                                     at 30° C.)                                                                    (mmol/g)                               ______________________________________                                        1    0.6      (gel formation)                                                 2    1        48,000    (244-285)                                                                             0.24   2.5                                    3    2         5,000    (202-240)                                                                             0.45   1.8                                    4    10        ˜700                                                                             77-85   >0.45  0.34                                    5*  10       ˜1000                                                                              85-125 >0.45  3.08                                   ______________________________________                                         *A mixture of phenylacetylene and vinylacetylene was used as chain            terminator.                                                              

EXAMPLE 5

Resins were prepared by dissolving the product of Example 1 in themonomer mixture to give a clear, viscous solution. Samples of thesolution weighing 6 to 10 grams were then cast into films, placed in anautoclave and cured at various temperatures and pressures, with heatingup to temperature and cooling being spread out over 8 hours to minimizeexotherms and fracture due to thermal shock. Thermal gravimetricanalyses were then performed with the results shown in Table 6.

                                      TABLE 6                                     __________________________________________________________________________    Resin Properties                                                                                         TGA Analysis*                                      Resin Composition (wt %)                                                                      Cure  Density                                                                            Temp. for                                                                            % Wt. Retention                             Monomer                                                                             Polymer                                                                            Graphite                                                                           Conditions                                                                          (g/cc)                                                                             10% wt. loss                                                                         600° C.                                                                    1000° C.                         __________________________________________________________________________    --    100  --   uncured                                                                             --   455    74.0                                                                              --                                      --    100  --   (a)   --   495    78.0                                                                              --                                      --    100  --   (b)   --   527    84.1                                                                              66.7                                    40    60   --   (c)   1.12 490    79.0                                                                              --                                      50    50   --   (c)   1.12 500    84.2                                                                              79.1                                    60    40   --   (c)   1.12 495    86.0                                                                              --                                      45    45   10   (d)   1.22 --     87.0                                                                              83.0                                    40    40   20   (d)   1.27 --     91.2                                                                              87.8                                    35    35   30   (d)   1.29 --     92.0                                                                              88.6                                    __________________________________________________________________________     *TGA heating rate 10° C./min                                           Cure Conditions:                                                              (a) 325° C., 6 h under N.sub.2                                         (b) 325° C., 30 h under N.sub.2                                        (c) 90° C., 16 h at 200 psig N.sub.2 ; 16 h at 200 psig N.sub.2        (d) Pyrolysis; 600° C., 4-6 h under Ar                            

The foregoing is offered primarily for purposes of illustration. It willbe readily apparent to those skilled in the art that numerousvariations, modifications and substitutions may be made in thematerials, procedural steps and conditions described herein withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. A polyphenylene polymer formed by thecopolymerization of the following monomers ##STR3## said polymercontaining at least a substantial portion of the vinyl groups of monomer(a) unreacted.
 2. A polyphenylene polymer in accordance with claim 1 inwhich at least about 50% of said vinyl groups remain unreacted.
 3. Apolyphenylene polymer in accordance with claim 1 in which at least about75% of said vinyl groups remain unreacted.
 4. A polyphenylene polymer inaccordance with claim 1 having an average molecular weight of from about2000 to about 100,000.
 5. A polyphenylene polymer in accordance withclaim 1 having an average molecular weight of from about 5000 to about50,000.
 6. A polyphenylene polymer in accordance with claim 1 in whichmonomer (b) comprises from about 30% to about 85% by weight of monomers(a) and (b) combined.
 7. A polyphenylene polymer in accordance withclaim 1 in which monomer (b) comprises from about 50% to about 70% byweight of monomers (a) and (b) combined.
 8. A polyphenylene polymer inaccordance with claim 1 in which the mole ratio of monomers (a) plus (c)to monomer (b) is from about 1 to about
 2. 9. A polyphenylene polymer inaccordance with claim 1 in which monomers (a) and (b) are independentlymeta-isomers, para-isomers or a combination thereof.
 10. A process forforming a polyphenylene polymer containing pendent vinyl groupssusceptible to crosslinking, said process comprising copolymerizing thefollowing monomers ##STR4## under conditions selected to leave at leasta substantial portion of the vinyl groups of monomer (a) unreacted. 11.A process in accordance with claim 10 in which said conditions areselected to leave at least about 50% of said vinyl groups unreacted. 12.A process in accordance with claim 10 in which said conditions areselected to leave at least about 75% of said vinyl groups unreacted. 13.A process in accordance with claim 10 in which said conditions areselected to provide said polymer with an average molecular weight offrom about 2000 to about 100,000.
 14. A process in accordance with claim10 in which said conditions are selected to provide said polymer with anaverage molecular weight of from about 5000 to about 50,000.
 15. Aprocess in accordance with claim 10 effected by the action of a Zieglercatalyst.
 16. A process in accordance with claim 15 in which saidZiegler catalyst is a complex of a titanium compound and an aluminumcompound.
 17. A process in accordance with claim 15 in which saidZiegler catalyst is a complex of a titanium halide and an alkylaluminumcompound.
 18. A process in accordance with claim 15 in which saidZiegler catalyst is a complex of titanium tetrachloride and atrialkylaluminum.
 19. A process in accordance with claim 15 in whichsaid Ziegler catalyst is a complex of titanium tetrachloride and R₃ Alwhere R₃ is a member selected from the group consisting of ethyl andisobutyl.
 20. A process in accordance with claim 10 in which monomer (b)comprises from about 30% to about 85% by weight of monomers (a) and (b)combined.
 21. A process in accordance with claim 10 in which monomer (b)comprises from about 50% to about 70% by weight of monomers (a) and (b)combined.
 22. A process in accordance with claim 10 in which the moleratio of monomers (a) plus (c) to monomer (b) is from about 1 to about2.
 23. A process in accordance with claim 10 effected by the action of aZiegler catalyst and in which said conditions include a temperaturemaintained within the range of about -10° C. to about 40° C.
 24. Aprocess in accordance with claim 10 in which monomers (a) and (b) areindependently either meta-isomers, para-isomers or a combinationthereof.
 25. A process in accordance with claim 10 further comprisingcopolymerizing vinylacetylene with said monomers.
 26. A polyphenyleneresin solution comprising a copolymer of(a) a monomer mixture comprising##STR5## and (b) a chain terminator having the formula ##STR6## saidcopolymer containing at least a substantial portion of the vinyl groupsof monomer (i) unreacted, and said copolymer dissolved in a furtheramount of said monomer mixture.
 27. A polyphenylene resin solution inaccordance with claim 26 in which said copolymer contains at least about50% of said vinyl groups unreacted.
 28. A polyphenylene resin solutionin accordance with claim 26 in which said copolymer contains at leastabout 75% of said vinyl groups unreacted.
 29. A polyphenylene resinsolution in accordance with claim 26 in which said copolymer has anaverage molecular weight of from about 2000 to about 100,000.
 30. Apolyphenylene resin solution in accordance with claim 26 in which saidcopolymer has an average molecular weight of from about 5000 to about50,000.
 31. A polyphenylene resin solution in accordance with claim 26in which monomer (ii) comprises from about 30% to about 85% by weight ofsaid monomer mixture.
 32. A polyphenylene resin solution in accordancewith claim 26 in which monomer (ii) comprises from about 50% to about70% by weight of said monomer mixture.
 33. A polyphenylene resinsolution in accordance with claim 26 in which the mole ratio of monomer(i) plus said chain terminator to monomer (ii) is from about 1 to about2.
 34. A polyphenylene resin solution in accordance with claim 26 inwhich said copolymer comprises from about 25% to about 75% by weight ofsaid polyphenylene resin solution.
 35. A polyphenylene resin solution inaccordance with claim 26 in which said copolymer comprises from about46% to about 60% by weight of said polyphenylene resin solution.
 36. Apolyphenylene resin solution in accordance with claim 26 in which saidcopolymer further incorporates vinylacetylene.